This invention relates to circuitry for control of the level of light output from a laser diode where light from the laser diode is directed to a resonant scanning mirror.
The operating characteristics of a laser diode are such that short duration transient spikes in voltage or current should be avoided to prevent damage to the laser diode. Consideration must also be given the light output versus current relationship which is quite steep. In addition, the use of a laser diode that is directed to the surface of a receptor member via a resonant scanning mirror requires that the output from the laser diode be changed in proportion to the velocity of the mirror, which is sinusoidal in nature, in order that the light energy is evenly received at the receptor. In the case where the laser diode is used in a printer, the laser diode must be turned on and off in accordance with the image to be printed.
Known arrangements for providing circuitry for modulating the output of a laser diode are based on current levels provided to the laser diode that are all additive in the same direction, or require changing of the duty cycle of digital control pulses. Another approach involves the combining of signals prior to any application to the laser diode. Such prior arrangements are complex in nature and are also subject to high frequency transients which are undesirable from the standpoint of protecting the laser diode and from the standpoint of image quality when the laser diode is used in a printer using electrophotographic technology.